Schematics of a two-chamber (flat anode) (A) and single-chamber membraneless (brush anode) (B) MEC. Bacteria (green ovals) grow on the anode and donate electrons but can also function as the biocatalyst on the cathode (dotted green ovals). Click to enlarge. Credit: ACS

A review of the materials, architectures, performance, and energy efficiencies of emerging microbial electrolysis cell systems (MECs) finds that MECs can efficiently convert a wide range of organic matter into hydrogen and are therefore a promising technology for renewable and sustainable hydrogen gas production from organic feedstocks.

However, the researchers conclude, there are a number of outstanding research questions that must be resolved for MECs to develop into a mature, commercial hydrogen production technology. The paper was published online 1 November in the ACS journal Environmental Science & Technology.

The review team included the two research groups who independently discovered several years ago that bacteria could be used to make hydrogen gas in an electrolysis-type process based on microbial fuel cells (MFCs). One group was led by Dr. Bruce Logan at Penn State, the other by Dr. René A. Rozendal at the University of Queensland (Australia).

MECs show high hydrogen yields and they need only a relatively small electrical energy input. Given these interesting properties, MECs could become viable technology to produce renewable hydrogen, provided a clean and renewable electricity input is used. Renewable hydrogen has many applications, the most
prominent ones being for transportation and industry.

Transportation fuels are currently responsible for about 20 to 25% of the global fossil fuel consumption. Because of climate change, and instabilities in the fossil fuel market, there is great interest in hydrogen as a transportation fuel (i.e., the hydrogen economy). Moreover, even without a hydrogen economy, there exists a large hydrogen demand.

In 2000, the global hydrogen consumption was already estimated to be 50 million tons per year, with about two-thirds used by the petrochemical industry. This hydrogen is used for upgrading fossil fuels and synthesis of industrial chemicals such as ammonia and methanol. Other industries that consume significant amounts of hydrogen include the food industry (saturation of fats and oils) and the metal
industry (as a reducing agent for metallic ores).

MECs can contribute significantly to these hydrogen demands by producing large quantities of hydrogen from renewable resources such as biomass and wastewaters. The MEC concept is now well proven, and significant advancements
have been made with respect to the performance in only a few years since its discovery.

—Logan et al. (2008)

Hydrogen production rate (Q) and energy efficiency
calculated from the electricity input and hydrogen output as a
function of the applied voltage using experimental data
(ηe real, based on heat of combustion) and theoretical
maximum energy efficiencies based on Gibbs free energy
(ηEΔG max) and heat of combustion (ηEΔH max). Click to enlarge. Credit: ACS

In a microbial fuel cell, bacteria oxidize organic matter and release carbon dioxide and protons into solution and electrons to an electrode (anode). (Earlier post.) The electrons flow from the anode through an electrical circuit to the cathode where they are consumed in the reduction of oxygen. Without oxygen, current generation is not spontaneous. However, if a small voltage (>0.2 V in practice) is applied between the anode and the cathode, hydrogen gas is produced at the cathode through
the reduction of protons. The system based on this latter process is termed a microbial electrolysis cell.

MEC systems are based on a number of components, each of which will require much additional investigation.

Microorganisms. Little is known about the composition of the microbial communities in MECs, the authors note. The only study of a community analysis of an MEC found that Pseudomonas spp. and Shewanella spp. were
present on the anode. Microorganisms are observed to be attached to the cathode, but to what extent they affect the function of the MEC is not clear.

Nor is it clear to what extent the operation of an MEC is affected by the inoculum source.

Another production issue to be resolved is that high concentrations of hydrogen gas also favors the growth of methanogens, reducing hydrogen gas production and contaminating the product gas with methane.

Materials. The anode material in a MEC can be the same as the material in a MFC—e.g., carbon cloth carbon paper, graphite felt, graphite granules or graphite brushes. Hydrogen production in an MEC occurs at the cathode. Because the hydrogen evolution reaction (HER) on plain carbon electrodes is very slow, a high overpotential is required to drive hydrogen production. To reduce this overpotential,
platinum is usually used as a catalyst.

There are many disadvantages to using platinum, the authors note, including
the high cost and the negative environmental impacts incurred during mining/extraction. Exploration of biocathodes are underway.

Other materials issues include membranes (although some MECs are membraneless), and tubing and gas collection systems.

Potential feedstock sources for MECs include wastewater (and wastewater treatment is a major potential application for MECs) and cellulosic biomass.

So far, MECs have achieved hydrogen production of up to 3.12 m3H2/m3 d with energy input of 0.8 V, values which are
in the same order as those of fermentation systems, according to the reviewers. MEC systems have reached a maximum current density of 186 A/m3. This, the authors note, is much lower than those in the more-extensively studied MFCs (5,600 A/m3, 10 A/m2), and thus it is likely
that with additional research, higher current densities will be achieved in MECs in the future.

The reviewers suggest that for MECs to become a mature hydrogen production technology, several research questions still need to be addressed:

More experience is required with real organic feedstocks containing complex
organic substrates such as polymeric and particulate substances;

Novel, more cost-effective chemical and/or biological cathodes need to be developed that show low potential losses and are not platinum-based;

Membrane pH gradients need to be eliminated, or membranes should
not be used in the reactor;

Methanogenic consumption of the hydrogen product needs to be prevented (in case of membrane-less MECs and/or MECs with a biocathode); and, most critically,

Comments

Joseph, You're wrong. Plants DO break down the H2O bond.

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or photosynthetic reactions (also called the Light Reactions) capture the energy of light and use it to make high-energy molecules. During the second stage, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture and chemically reduce carbon dioxide (CO2) (also called carbon fixation) to make the precursors of carbohydrates.

In the light reactions no CO2 gas is present, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP Synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases a dioxygen (O2) molecule:
2 H2O + 2 NADP+ + 2 ADP + 2 Pi + light → 2 NADPH + 2 H+ + 2 ATP + O2

In the Light-independent or dark reactions the enzyme RuBisCO captures CO2 from the atmosphere and in a process that requires the newly formed NADPH, called the Calvin-Benson Cycle, releases three-carbon sugars;
3 CO2 + 9 ATP + 6 NADPH + 6 H+ → C3H6O3-phosphate + 9 ADP + 8 Pi + 6 NADP+ + 3 H2O, which are later combined to form sucrose and starch.

Photosynthesis may simply be defined as the conversion of light energy into chemical energy by living organisms and as an energy converter it's very inefficient; it uses only 1% of the entire electromagnetic spectrum, and 2% of the visible spectrum. Even the best plants convert light into chemical energy with a maximum photosynthetic efficiency of approximately 6% (and use a lot of that up to maintain their own life functions), add in the process of converting biomass to electricity, which is about 50% (and I'm being generous). By comparison solar panels convert light into electric energy at a photosynthetic efficiency of approximately 10-20%.

BTW Yes we should use more water to get hydrogen. Why? Because currently when industry needs bulk hydrogen they get it from breaking down methane(CH4) in a process that releases CO2 into the atmosphere. We here at the GCC see that as a bad thing.

Nice post about photosynthesis. However, I took you for a really young guy when it comes to energy and the environment. While Joseph is confused about science, ai_vin approach is simplistic.

The role of plants in nature is to provide food, fiber, and energy storage. It may be inconceivable to ai_vin that the human race survived several ice ages without incorrectly calculating the efficiency of solar PV. Huddled in cave hoping your nuts and dried wild grain was popular for a million years. Give me a Franklin stove, a wood pile, beans, rice, good cheese, and wine and you are set for the mother of all ice storms. ai_vin will have to settle for PV panels and a million $s in batteries.

In you travels have you been to North Africa. Notice how they keep homes cool. The principle is thermal mass. Solar PV in the Southwest US, institutional stupidity along with the rest of Title 24 in California.

The point here is that PV is not very useful.

So ai_vin, I hope we do not have this talk about comparing PV and biomass. You may also think about ganging up with Bob on baseless demeaning people of people.

Sorry I did not post my name. Since you failed understand my point I will repeat it. You are not as wise as you think you are. On the topic of energy and the environment, you are very close to Joseph. Neither ai_vin nor Bob have demonstrated any understand of the topic to suggest that they are smart.

While ai_vin did correct Joseph on a point of science, ai_vin then proceeded to demonstrate the same level of understanding as Joseph by discussing efficiency.

I have proven that thermal mass is a very efficient way to keep a house comfortable in the summer and in the winter without using electricity. I used a slide rule to do the calculations before building. In the history of man, electricity is rather a new thing. Computers are even newer.

It would be interesting to see how you power your computer at night with PV.

I will say anything to stop any Tech that would cause the Megalith called LA to use any more water than they already do. LA has turned every valley within 500 miles into a dust bin. The Colorado River is barely a trickle by the time it reaches Yuma. California has had 6 desalinization plants go online in the past two years and its never enough, water restrictions every year.

Now you want them to have another reason to use massive amounts of water. That's Insane!!!!

I said to adopt water electrolysis for propelling cars, trucks, airplanes, machineries, electrical generation, boats, etc. The water electrolyser providing hydrogen gas from water must be installed permanentatly near the engine for practicallity and only water is needed to fuel the system.

And stop arguing about science. I need to buy a car powered by water and there is no one available in regular commerce because you spread fear and confusion.

The childs working on the water-powered car actually, around 1000 of them are ridiculously slow pokes. They must argue with numerous stupids persons because they don't even know the basics of how a conventional ice engine work even if it's invented since 100 years, LOL.
First they don't stock a little pressurised quantity of hho gas near the engine, it's then impossible to follow the need in fuel of the engine that work from idle to high rpm and load conditions. How can you feed and inject hho gas at the right amount with precision needed if you don't even have a capable devise to do it and no electronic metering of rpm, load, ignition timing, hydrogen pressure, etc. Then the water electrolyser must be capable of producing gas in quantity to sustain high load high rpm operations. Try the electric and electrode configuration of stan meyer or d.dingel or hypowerfuel from canada. Don't forget hydrogen explode then ignition timing must be done at top dead center or even after and a fine mist of water inject at the correct quantity for all rpm and load assist in expanding the pressure in the engine and this water can be hot to expand more in the engine but not too hot to impede the compression cycle. Go to H.A.W site in japan to learn about this, it help the engine to produce 50% more power then gasoline for a same engine size.